As radio apparatus like mobile phones, etc. come into wide use and have a wider range of application, antennas having a broader frequency range are required more than ever for radio apparatus. For instance, it is thought that an ultra-high frequency (UHF) band as broad as a couple of hundred megahertz (MHz) is required for receiving terrestrial digital television broadcast. In order that a radio apparatus using a single antenna for downsizing is adapted for two or more standards of wireless local area networks (WLAN) of different frequencies, the antenna has to cover both 2.4 gigahertz (GHz) and 5.2 GHz bands.
A multi-resonant antenna applicable to WLANs is disclosed in Japanese Patent Publication (Kokai), No. 2003-46318. The multi-resonant antenna includes an antenna element formed by a linear or band-shaped conductor one end of which is fed at a feeding point on and surrounded by a ground plane and another end of which is grounded on the ground plane. The antenna element is loaded with an impedance element so that the multi-resonant antenna resonates at a frequency determined by a length of the linear or band-shaped conductor and resonates at another frequency determined by a value of the impedance element.
An antenna including an antenna element formed by a linear or plate-shaped conductor loaded with a reactance element for multi-resonance is disclosed in Japanese Patent Publication (Kokai), No.2004-40596, and is called here a reactance-loaded antenna. The antenna element of the reactance-loaded antenna is divided by the reactance element by a ratio that determines a resonant frequency of the reactance-loaded antenna.
A majority of radio apparatus adopt built-in antennas these days. In order to select a type of built-in antennas, it is necessary to examine candidates of built-in antennas from viewpoints of size, inefficiency of radiation caused by return currents, necessity of balance-to-unbalance transformation, etc.
As one of the candidates that may somehow satisfy the necessity from the above viewpoints, known is a half-wavelength T-type monopole antenna. One example of that type of antenna is disclosed in:    Sekine, S. and Shoki, H., “Characteristics of T-type monopole antenna with parallel resonance mode”, IEICEJ. Trans. Vol. J86-B, No. 2, pp. 200-208, February 2003 (in Japanese).
The above example of the T-type monopole antenna is configured that lengths of a left half and a right half thereof are unequal so that the antenna may be resonant in a parallel resonance mode and may cope with both downsizing and efficiency of radiation by increasing input impedance.
The multi-resonant antenna or the reactance-loaded antenna described above may be multi-resonant by being loaded with a reactance element located at a middle portion of the antenna element, i.e., somewhere between both ends of the antenna element. In a case where a resonant frequency needs to be changed or tuned, however, a value of the reactance has to be adjusted. Hence, there is a problem that the above antennas may not be suitable for an application that needs a significant change of the resonant frequency, e.g., WLANs.
The parallel resonance mode of the T-type monopole antenna described above may be effective depending on a configuration of radio apparatus. The T-type monopole antenna, however, may not be suitable for a radio apparatus a feeder system of which is designed to match the input impedance of the antenna at a series resonant frequency of the antenna, as the increase of the input impedance in the parallel resonant mode is likely to cause a mismatch.